Flue gas gypsum is made in the flue gas desulfurization of power plant exhaust gas. The method comprises treating the flue gas in a scrubbing tower with a scrubbing water suspension containing calcium oxide, calcium hydroxide or calcium carbonate. Scrubbed sulfur dioxide is then converted by oxidation with atmospheric oxygen fed to the system and by crystallization to calcium sulfate dihydrate crystals. A recycle flow is drawn from the scrubbing tower sump and is delivered by spray nozzles to the scrubbing tower.
Furthermore an additional crystal suspension stream is continuously drawn from the scrubbing tower sump. The drawn stream is split by a cyclone separator into a lower run-off resulting in a crystal slurry and an upper run off or decantate fed again to the scrubbing tower sump and having a finely divided solid component. The lower run off is dewatered, flue gas gypsum separated and the clear effluent is returned to the scrubbing tower.
In this process the scrubbing tower sump pH value is adjusted to a pH of 4 to 6. The delivery of calcium oxide, calcium hydroxide or calcium carbonate can occur at different locations in this process. Thus, as described in VGB Kraftswerkstechnik 63 (1983) No. 4, pages 335 to 344, a slurry of limestone (calcium carbonate) is fed to the upper portion of the scrubbing tower while calcium hydroxide in an aqueous suspension (milk of lime) is delivered to the scrubbing tower sump. Advantageously the flue gas is scrubbed in a counterflow. The flue gas can be fed in a continuous flow to the sprayed scrubbing water suspension.
In the known process only the solid content is constantly adjusted in the scrubbing tower sump since the crystal suspension is withdrawn by pump continuously or periodically from the scrubbing tower sump and scrubbed or purified. The separated fluid is fed to the scrubbing tower. The proportion of fine crystals present is extremely small and is approximately 1 weight % in that size range.
The flue gas gypsum made according to this known process is large grained and can be dewatered with a residual moisture content of about 6 to 7 weight %. Grain size of the flue gas gypsum so made is larger than the grain size spectrum of flue dust or fly ash.
Both the grain distribution and the water binding capacity are controllable only to a small extent. The large grain size and the small crystal surface area are disadvantageous in making disposable mixtures with flue dust or fly ash. Particularly, the solidification characteristics is unsatisfactory of a mixture of flue gas, gypsum and scrubbing water suspension components.